University of Chicago researchers hunt for proposed particles that could explain quirks of the universe.
A team of researchers at the University of Chicago recently embarked on the search of a lifetime—or rather, a search for the lifetime of long-lived supersymmetric particles.
Supersymmetry is a proposed theory to expand the Standard Model of particle physics. Akin to the periodic table of elements, the Standard Model is the best description we have for subatomic particles in nature and the forces acting on them.
But physicists know this model is incomplete—it doesn’t make room for gravity or dark matter, for example. Supersymmetry aims to complete the picture by pairing each Standard Model particle with a supersymmetric partner, opening up a new class of hypothetical particles to detect and discover. In a new study, UChicago physicists have uncovered limitations for what properties these superpartners, if they exist, could have.
“Supersymmetry really is the most promising theory we have for solving as many problems as possible in the Standard Model,” said Tova Holmes, assistant professor at the University of Tennessee, Knoxville, who worked on the experiment as a postdoctoral researcher at UChicago. “Our work fits into a larger effort at the Large Hadron Collider to reconsider how we search for new physics.”
The Large Hadron Collider, located in Europe at CERN, accelerates protons to nearly the speed of light before forcing them to collide. These proton-proton collisions produce a slew of additional particles where researchers hope to find new physics.
“But at the Large Hadron Collider, new physics events are extremely rare and difficult to identify in the debris of colliding particles,” said Prof. Young-Kee Kim, chair of the UChicago physics department and co-author of the study, an effort led entirely by women.
The UChicago team searched for the production of sleptons—hypothesized superpartners of the existing electron, muon, and tau leptons—using data collected in ATLAS, a particle detector at CERN. In the tested supersymmetry model, sleptons are theorized to have long lifetimes, meaning they can travel far before decaying into something detectable by ATLAS.
“One of the ways we can miss new physics is if the particle doesn’t decay promptly when it’s produced,” said Holmes. “Typically, we’re blind to long-lived particles in our searches, because we basically cut out anything that doesn’t look like a standard prompt decay in our detector.”
Sleptons are expected to eventually decay into their regular lepton partners. But unlike conventional decays, these leptons will be displaced, meaning they won’t point back to the original proton-proton collision point. It was this unique feature that physicists were hunting for.
In four years of collected ATLAS data, however, UChicago researchers found no displaced lepton events. That lack of discovery allowed them to set what is called a limit, ruling out a range of masses and lifetimes that long-lived sleptons might have.
“We are at least 95% sure that, should a slepton in this model exist, it doesn’t have the masses and lifetimes in the shaded portions of this plot,” said Lesya Horyn, newly minted PhD from UChicago who recently completed her dissertation on this measurement.
Does a null result disappoint the team? Not at all.
“Finding nothing tells you so much,” Horyn said. Knowing that long-lived sleptons don’t have certain masses and lifetimes informs researchers on where to focus future searches.
“Supersymmetry really is the most promising theory we have for solving as many problems as possible in the Standard Model.”
— Tova Holmes, assistant professor at the University of Tennessee, Knoxville
“From my point of view, this search was the number one thing theorists were calling out to have covered,” Holmes said. “It seemed like we could do it—and we did!”
The outcome has energized the team to push the boundaries even further. At some point in the next decade, the Large Hadron Collider will enter its periodic shutdown, leaving ample time for ATLAS hardware to be upgraded.
“This was a first pass at the analysis, so there are definitely places to improve,” Horyn said.
One pressing upgrade will be a revamp of the trigger system, which selects whether events should be saved or thrown away. The trigger is currently optimized to store decays from short-lived particles, not the long-lived sleptons central to this supersymmetry search.
More immediate improvements can be made without waiting for the shutdown.
“Future steps might include searching for the same model using more robust data from the next runs of the Large Hadron Collider,” said Xiaohe Jia, a graduate student at Harvard who worked on the experiment as a UChicago undergrad. Another route to explore, she said, could be using similar techniques to expand the long-lived particle search beyond just sleptons.
For now, the completion of the Standard Model remains a mystery, but the team is proud to have led a first search for this supersymmetry model in ATLAS.
“Discovering new physics is like finding a needle in a haystack,” Kim said. “Although we did not see anything in the current data, there is great opportunity for the future!”
Reference: “Search for displaced leptons in √ s = 13 TeV pp collisions with the ATLAS detector” by The ATLAS Collaboration, 6 October 2020, ATLAS CONF Note.
Funding: National Science Foundation.
… so, why not use the naturally speedy particles coming from out of the space…
…HoW? Well, you figure out that, because you have a way more regular pay check than mine…
“Supersymmetry really is the most promising theory we have for solving as many problems as possible in the Standard Model”.
This is an arguable claim now that naturally expected thermal dark matter candidates of supersymmetric string theory WIMPs hasn’t been found in the LHC range.
“Obtaining the correct abundance of dark matter today via thermal production requires a self-annihilation cross section of … which is roughly what is expected for a new particle in the 100 GeV mass range that interacts via the electroweak force. Because supersymmetric extensions of the Standard Model of particle physics readily predict a new particle with these properties, this apparent coincidence is known as the “WIMP miracle”, and a stable supersymmetric partner has long been a prime WIMP candidate.”
[“Weakly interacting massive particles”, Wikipedia]
The alternative to such naturalness is apparent finetuning which is possibly found in existing physics, not particle physics alone but within cosmology. The observed best candidate for the inflation field is a simple scalar quantum field in the Planck collaboration 2018 survey. It is unstable and would result in the correct vacuum energy (dark energy) density by way of Weinberg’s anthropic multiversum.
The eBOSS galaxy survey collaboration 20 year result find it a viable candidate.
“Nevertheless, the observed consistency with flat ΛCDM at the higher precision of this work points increasingly towards a pure cosmological constant solution, for example, as would be produced by a vacuum energy finetuned to have a small value. This fine-tuning represents a theoretical difficulty without any agreed-upon resolution and one that may not be resolvable through fundamental physics considerations alone (Weinberg 1989; Brax & Valageas 2019). This difficulty has been substantially sharpened by the observations presented here.”
[“THE COMPLETED SDSS-IV EXTENDED BARYON OSCILLATION SPECTROSCOPIC SURVEY: COSMOLOGICAL IMPLICATIONS FROM TWO DECADES OF SPECTROSCOPIC SURVEYS AT THE APACHE POINT OBSERVATORY”, arxiv]
When applied to non-WIMP dark matter – solely gravitationally inetracting – quantum field physics sets a finetuning condition on a similar unstable simple scalar quantum particle field.
“For example, for singlet scalar dark matter, we find a mass range 10^-3 eV <= m_fi <= 10^7 eV . The lower bound comes from limits on fifth force type interactions and the upper bound from the lifetime of the dark matter candidate."
[ https://www.sciencedirect.com/science/article/pii/S0370269321000083?via%3Dihub ]
And finely we have the finetuning in the scalar quantum field Higgs sector, which is not as simple with its 4 fields of a doublet boson. It couples to several gauge bosons which makes it stable in the region from the ~ 10^11 GeV inflation field down to the electroweak symmetry breaking range that gives particles mass at the ~ 250 GeV range probed in the LHC. But it is still finetuned in respect to the Higgs mass and the ability to produce atoms of complex elements.
["The anthropic principle and the mass scale of the Standard Model", arxiv]
As a consequence of inflationary physics it could be that in our case physics inhabits 3 different energy scales, the ~ 10^11 GeV of inflation, the ~ 10^2 GeV of normal matter and the ~ 10^-2 GeV of dark matter (assuming it too is "just so" picked like dark energy and normal matter). The amount of finetuning, i.e. the cancellation of large factors to allow such energies, would be 10^-120 for the vacuum energy, 10^-42 for the dark matter* and 10^-34 for the normal matter ["Fine-tuning versus naturalness", Symmetry Magazine], or ~ 10^-196 in total.
That's a lot of universes for every habitable universe. But the upshot is that no new physics is required, if the already discovered physics behave as expected.
*Assuming the same type of coupling in quadrature to gravity and referencing the 10^19 GeV Planck scale, i.e. O(10^-4) = O(10^38) – O(10^38) cancellations. ["Naturalness, Wilsonian Renormalization, and “Fundamental Parameters” in Quantum Field Theory", CERN]
… some evidence to that story, which I will not read, because I heard it from 1.000.000 mouth and it is same old, same old, same old…
Supersymmetry with pairing of particle is present enrout in the evolution mechanism of Milkyway,the the Super massive black hole at the centre – stars evolved and ending at the nagnetic field with Flux containing neutrinos and antineutrinos.We have to quest the ù niversal process complitly,have to get right idea on map,structor and cronological sequence of stars and every entity that preset upto the end right on magnetic flux.
Supersymmetry and pairing of particles seems to be obvious. Our Milkyway evolved the Supemassive black hole at centre,then the stars and ending on the Magnetic Field connected to the Flux,generating neutrinos and antineutrinos. The simple thing we have to make it that to grsp the idea with right cro nological order the process of fòrmation of all above alongwith right map emphasizing on the ordered structure.